The present invention relates generally to gas turbine engines, and, more specifically, to land vehicle turbine engines.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases from which energy is extracted by downstream turbine stages. A high pressure turbine (HPT) immediately follows the combustor and is joined by a first rotor or shaft to the upstream compressor which typically includes multiple stages. A low pressure turbine (LPT) is disposed downstream of the HPT and produces output power for a second rotor or driveshaft.
In a typical turbofan engine, the LPT is joined to a large fan in front of the compressor for producing propulsion thrust for powering an aircraft in flight. In a land or marine-based engine, the LPT may be joined to an external device for providing power thereto. The engine may be configured for powering a ship, a land vehicle, or an electrical generator in typical applications.
Although the gas turbine engines used in these various applications are fundamentally similar in configuration, they nevertheless must be specifically tailored for those different applications and the different problems associated therewith.
For example, a gas turbine engine configured for a military vehicle, such as a battle tank, must be compact in configuration, readily accessible for field replacement of typical parts, and efficient in operation, with minimal exhaust emissions. These are just several of many competing design objectives for vehicle engines which differ from those associated with aircraft engines.
Vehicle gas turbine engines therefore place a premium on size, weight, and complexity of the engine for maximizing operating range of the vehicle and durability of the engine. The engines must be designed to start and operate in cold or hot environments between sea level and high altitude. Starting is particularly difficult because battery powered, low energy starters must be used to save vehicle weight, and starting requires acceleration of the turbine and compressor rotor to a major percentage of maximum rotor speed representing steady state idle. Turbine rotors may operate at tens of thousands of revolutions per minute (RPM), and steady state idle is typically well above 50 percent maximum rotor speed.
The vehicle turbine engines may be operated with alternate fuels and must operate at high combustion efficiency at very low fuel-to-air ratios just above flameout. And, the accel-to-idle starting of the engine must be free of white smoke emissions, which are typically created when unreacted, evaporated fuel condenses in the exhaust flow. This problem is further increased when a recuperator heat exchanger is used in the engine for preheating compressor air for the combustor by using the hot exhaust gases from the turbine. The recuperator acts as a reservoir for any raw fuel which is discharged thereto due to incomplete combustion, particularly during starting.
Furthermore, efficient fuel atomization is required for achieving efficient combustion, and fuel atomization is affected by the type of fuel injectors and air mixing system.
For example, relatively simple airblast fuel injectors are conventional and cooperate with surrounding air swirlers mounted to the dome end of the combustor for producing fuel and air mixtures. Fuel atomization is affected by the flow rate and pressure of the swirler air which are relatively low during engine starting.
In contrast, fuel-pressurizing injectors, such as the common duplex fuel injector, are configured for using high pressure fuel for finely atomizing the fuel during starting or above idle operation of the engine. However, such pressurizing injectors are more complex than airblast injectors and require a more powerful fuel pump for providing sufficient fuel pressure during starting and above idle performance.
Accordingly, it is desired to provide an improved combustor for a vehicle gas turbine engine, and corresponding method of starting thereof.